This invention relates to feeders and more particularly to feeders for handling highly abrasive materials
Volumetric and gravimetric feeders have been widely used for controlling the feed rate of materials used in various industrial applications. While a wide variety of feeder types have been employed, the choice is normally governed by the properties and characteristics of the material whose feed rate is to be controlled and the requirements of the specific application. For example, the material may vary in granulometry from a few microns in particle size up to 10 or 20 millimeters or more. In addition, the material particle may be uniform in size and shape or irregular or may vary in abrasiveness. Another variable is the feed accuracy required by the specific application. In some cases, relatively accurate and uniform flow rates may be required while other applications may permit fluctuations of up to 5 or 10 percent from the nominal required value.
Examples of prior art volumetric and gravimetric feeders include Archimedean screws, multiple rotary vane and vibratory types. Various problems have been encountered in employing prior art feeders resulting from a combination of the characteristics of the material being fed and the constraints or requirements imposed by the specific application. These problems include inaccurate or irregular flow rates usually effected by changes in material granulometry or fluidity, changes of material head column at the inlet of the feeder and changes of relative pressure or air currents across the feeder in applications where the feeder is connected to processes or ducts under pressure.
Another problem with prior art feeders is the abrasion of apparatus surfaces, shaft seals and bearings where abrasive materials are handled. This problem is compounded in applications where highly accurate flow rates are required which necessitates relatively small clearances between rotating members, such as screws or vanes, and the casing of the apparatus resulting in greater abrasive action between the material and the feeder component surfaces.
Some prior art feeders are also susceptible to self-destruction by overload when the discharge material exit from the feeder is blocked. This occurs when the apparatus and drive train forces material against a dead head or blocked exit.
One application where gravimetric or volumetric feeders are employed for feeding abrasive materials at a controlled rate is in the treatment of emissions from aluminum production facilities. In particular, the production of aluminum by various well known processes generates effluent gases such as hydrogen flouride and other flouride compounds, carbon monoxide, carbon dioxide, sulfur dioxide, hydrocarbons and entrained filing divided solid particles of aluminum, carbon and other materials. These emissions are commonly vented by means of specially designed hoods and ducts to gas cleaning systems to prevent the discharge of pollutants into the atmosphere and for the recovery of valuable flouride and alumina contained in the gases.
In one well known type of emission control system, metal grade alumina is employed for the adsorption and removal of hydrogen fluoride and other fluoride compounds as well as the entrapment of hydrocarbons from gases evolved in the aluminum manufacturing process. The principle advantage of using alumina for pollution control and material recovery is that the alumina along with the recovered fluoride can readily be returned to the reduction pots or cells, for use in the aluminum recovery process.
In one type of fume treatment system, alumina is fed at a controlled rate into a horizontal or vertical duct through which the waste gases are conducted. The alumina particles are dispursed into the waste stream for the adsorption of fluorine compounds. One such apparatus for cleaning waste gases from aluminum production facilities is disclosed in U.S. application Ser. No. 470,787, filed Mar. 1, 1983, now U.S. Pat. No. 4,501,599.
One major problem in such prior art systems results from the fact that the alumina is highly abrasive. As a result, rapid wear is encountered not only in the gas cleaning system itself but in the apparatus employed for feeding alumina particles at the desired rate into the gas cleaner.